NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOX emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOX emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOX emissions. Another set of approaches remove NOX from the vehicle exhaust. These include the use of lean-burn NOX catalysts, selective catalytic reduction (SCR) catalysts, and lean NOX traps (LNTs).
Lean-burn NOX catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOX in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. A reductant such as diesel fuel must be steadily supplied to the exhaust for lean NOX reduction, introducing a fuel economy penalty of 3% or more. Currently, peak NOX conversion efficiencies for lean-burn NOX catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOX by ammonia. The reaction takes place even in an oxidizing environment. The NOX can be temporarily stored in an absorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOX reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
LNTs are devices that adsorb NOX under lean conditions and reduce and release the adsorbed NOX under rich conditions. An LNT generally includes a NOX adsorbent and a catalyst. The adsorbent is typically an alkali or alkaline earth compound, such as BaCO3 and the catalyst is typically a combination of precious metals including Pt and Rh. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOX adsorption. In a reducing environment, the catalyst activates reactions by which hydrocarbon reductants are converted to more active species, the water-gas shift reaction, which produces more active hydrogen from less active CO, and reactions by which adsorbed NOX is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time-to-time to regenerate (denitrate) the LNT.
Regeneration to remove accumulated NOx may be referred to as denitration in order to distinguish desulfation, which is carried out much less frequently. The reducing environment for denitration can be created in several ways. One approach uses the engine to create a rich exhaust-reductant mixture. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. A reducing environment can also be created by injecting a reductant into lean exhaust downstream from the engine. In either case, when valves are not used, a portion of the reductant is generally expended to consume excess oxygen in the exhaust. The reducing agent reacts with oxygen and substantially consumes it. The reactions between reductants and oxygen can take place in the LNT, but it is generally preferred for the reactions to occur in a catalyst upstream from the LNT so that the heat of reaction does not cause large temperature increases within the LNT at every regeneration. To lessen the amount of excess oxygen and reduce the amount of reductant expended consuming excess oxygen, the engine may be throttled, although such throttling may have an adverse effect on the performance of some engines.
WO 2004/090296 describes a diesel automotive exhaust aftertreatment system with a fuel reformer configured within an exhaust line upstream from LNT and SCR catalysts. The reformer has a high thermal mass. The reformer uses Pt and Rh to produce syn gas from diesel fuel at exhaust gas temperatures. For the reformer to be operative at exhaust gas temperatures, a relatively large amount of catalyst must be used to provide enough catalyst activity. The reformer both removes excess oxygen and converts the diesel fuel reductant into more reactive reformate.
U.S. Pat. Pub. No. 2004/0050037 (hereinafter “the '037 publication”) describes a different type of fuel reformer placed in the exhaust line upstream from an LNT. The reformer includes both oxidation and steam reforming catalysts. Pt and/or Pd serves as the oxidation catalyst. Rh serves as the reforming catalyst. The inline reformer of the '037 publication is designed to be rapidly heated and to then catalyze steam reforming. Temperatures from about 500 to about 700° C. are said to be required for effective reformate production by this reformer. These temperatures are substantially higher than typical diesel exhaust temperatures. The reformer is heated by injecting fuel at a rate that leaves the exhaust lean, whereby the injected fuel combusts to generate heat. After warm up, the fuel injection rate is increased and or the oxygen flow rate reduced to provide a rich exhaust. U.S. Pat. No. 7,213,395 describes an exhaust aftertreatment system comprising a similar reformer-LNT system, but with an SCR reactor configured downstream from the LNT.
Designing the fuel reformer to heat and operate at least partially through steam reforming reactions as opposed to operating at exhaust stream temperatures reduces the catalyst requirement, increases the reformate yield, and reduces the amount of heat generation. In principal, if reformate production proceeds through partial oxidation reforming as in the reaction:CH1.85+0.5O2→CO+0.925H2  (1)1.925 moles of reformate (moles CO plus moles H2) could be obtained from each mole of carbon atoms in the fuel. CH1.85 is used to represent diesel fuel having a typical carbon to hydrogen ratio. If reformate production proceeds through steam reforming as in the reaction:CH1.85+H2O→CO+1.925H2  (2)2.925 moles of reformate (moles CO plus moles H2) could in principle be obtained from each mole of carbon atoms in the fuel. In practice, yields are lower than theoretical amounts due to the limited efficiency of conversion of fuel, the limited selectivity for reforming reactions over complete combustion reactions, the necessity of producing heat to drive steam reforming, and the loss of energy required to heat the exhaust. Nevertheless, the benefits are sufficient that a low thermal mass reformer that must be preheated to operate effectively is preferred over a large thermal mass reformer that does not require preheating.
In spite of advances, there continues to be a long felt need for an affordable and reliable diesel exhaust aftertreatment system that is durable, has a manageable operating cost (including fuel penalty), and reduces NOX emissions to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations.